Warning on nightly about implicit imports

[sfackler]

extern crate enumflags;
#[macro_use]
extern crate enumflags_derive;

#[derive(EnumFlags, Copy, Clone)]
enum Foo {
    A = 1,
}

warning: cannot find type `Foo` in this scope
 --> src/main.rs:5:10
  |
5 | #[derive(EnumFlags, Copy, Clone)]
  |          ^^^^^^^^^ names from parent modules are not accessible without an explicit import
  |
  = note: #[warn(proc_macro_derive_resolution_fallback)] on by default
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #50504 <https://github.com/rust-lang/rust/issues/50504>